1. Field of the Invention
The present invention relates to a current-source power converting apparatus with self-extinction devices, and more particularly to the power converting apparatus which includes an AC-DC converter for converting an alternating current power furnished from an AC power source into a direct current power and a DC-AC inverter connected with the AC-DC converter through a direct current reactor for re-converting the direct current power into an alternating current power to supply the reconverted power for a load, and which is suitable for continuing operation of the DC-AC inverter even at the time of failure of the AC power source.
2. Description of the Related Art
There is a demand that, even if an AC power source fails, a power converting apparatus fed from the AC power source must continue to operate for a while. For example, a driving motor for an elevator is required to continue to operate until an elevator cage running at that time reaches the most neighboring floor safely. The power converting apparatus supplying such a driving motor with the electric power has to continue the feeding of the necessary power, the performance thereof being more or less derated.
Usually, two types of the power converting apparatus are known; one type is a so called voltage-source type and the other a current-source type. The former has been used rather more frequently from the reason as follows.
In the voltage-source power converting apparatus, an AC-DC converter included in such apparatus has not been required to be capable of controlling its output DC voltage. The AC-DC converter was sufficient only to output the DC power of the constant voltage, because the control of the voltage applied to a load can be easily realized by a DC-AC inverter connected to the AC-DC converter. Accordingly, at the time of failure of an AC power source, the AC-DC converter is replaced by a battery which can supply the DC-AC inverter with the DC power of the constant voltage, and the voltage of the AC power supplied for the load is controlled by the usual control method of the DC-AC inverter.
However, when the voltage-source power converting apparatus conducts a regenerative operation, it becomes necessary to provide another converter exclusively used for the regenerative operation. On the other hand, in the current-source one, the power converting apparatus can achieve the regenerative operation by only the gate control of one converter without any further converter. Therefore, the current-source power converting apparatus is used, when the regenerative operation is required.
Contrary to the case of the voltage-source type, however, a DC-AC inverter used in a current-source power converting apparatus is very difficult to control the voltage of its output AC power. Such control has been scarcely feasible in a practical use. Therefore, an AC-DC converter connected with the DC-AC inverter through a DC reactor has to fill the role of voltage control of the AC power supplied for a load as the final output of the current-source power converting apparatus. That is to say, the voltage of the DC power furnished for the DC-AC inverter has to be controlled by the AC-DC converter. Accordingly, the DC-AC inverter can not continue to operate by merely substituting a battery for the AC-DC converter at the time of failure of an AC power source. Such substitution of the battery for the converter made possible the continuous operation of the inverter at the time of failure of the AC power source in a case of the foregoing voltage-source power converting apparatus.
Now, in a conventional example, a current-source power converting apparatus employing self-extinction devices utilizes the DC short circuiting mode of operation for a usual control of the DC voltage by means of the AC to DC conversion. In order to make such a power converting apparatus operate continuously at the time of failure of an AC power source, a battery is connected to the apparatus and the control of DC voltage must be performed by the DC to DC transformation. Since, however, the DC to DC transformation necessiates the provision of a freewheel diode, the arrangement of a main circuit of the usual converter is not suited for control of the DC voltage. Further, in case the freewheel diode is connected to the output end of the AC-DC converter, there arises a defect that the regeneration becomes impossible in the usual operation.